This project seeks to investigate spatial mobility across neighborhoods as well as relationships between Global Positioning System (GPS)-defined activity space neighborhoods and HIV risk among young men who have sex with men (MSM) in the New York City metropolitan area, through the use of innovative methodological approaches including real-time geospatial methods and geo-located Twitter posts. We will randomly enroll 250 young MSM in the NIH-funded P18 Cohort Study in the proposed study to address the aims of the research. Eligibility requirements include report having had sex with another male in the past 6 months; HIV- seronegative; self-report no restrictions to usual physical activity; and willingness to complete a two-week GPS protocol. Participants will wear the GPS device following protocols we have used in our previous feasibility research projects. Findings from the proposed research will impact HIV prevention intervention activities. First, this research will inform specific neighborhood-level policy interventions. For example, increasing community efforts to combat lesbian, gay, bisexual and transgender (LGBT) hate crime neighborhood rates through increased local police attention in high-crime locations may be an HIV prevention intervention. Second, given that we will know the travel patterns of young MSM, we can identify geographic locations for HIV testing/prevention interventions, which is an advancement of the literature as such interventions are not often spatially targeted. Finally, this research will also facilitate identifying geographic locations for recruiting young MSM in research studies (an improved method of venue-based sampling), in this understudied group.